For injection control duration values that are greater than a minimum time, injectors of this type have a flow rate characteristic comprising a zone that is substantially linear, which zone is defined by a gain corresponding to its slope and by an offset at the origin corresponding to a floor control duration for which the injected mass is zero, and located where the extension of the linear zone towards the origin of injection control durations intersects the abscissa axis representing injection control durations on a graph where injected masses are plotted up the ordinate axis. For small values of injection control durations, the characteristic also comprises a non-linear zone between the offset and the linear zone. The local gain thus corresponds to the local slope at any point along the curve representing the flow rate characteristic of the injector.
Injectors of this type are generally qualified by their manufacturers in terms of a nominal or theoretical flow rate characteristic having nominal theoretical linear and non-linear zones initially stored in the engine control unit, e.g. in the form of a theoretical offset and a theoretical gain for the linear zone, and at least one theoretical table or chart or mathematical relationship for the non-linear zone.
Electrically-controlled fuel injectors of this type can be fitted to diesel engines or to controlled ignition engines, and they can be mounted in direct or indirect injection feed circuits, with or without fuel being returned from downstream to upstream around the pump.
It is known that the injectors used to enable the engine control unit to inject a predetermined quantity of fuel present both mutual dispersions and variations over time in their flow rate characteristics, and as a consequence injecting a given mass of fuel requires different injection control durations depending on the injector being controlled and on its aging. The dispersions in the characteristics of injectors are the result of manufacturing tolerances applicable to the physical components of the injectors, and thus to dispersions in their dimensions and physical characteristics, in particular in the number and the diameter(s) of the injection orifices of injectors, in their orientations, in the resilient characteristics of their springs, etc. . . . , and the variation of the flow rate characteristics of injectors over time is a result in particular of the physical components of the injectors aging.
Furthermore, the great majority of systems for controlling direct or indirect injection and fitted to internal combustion engines of motor vehicles provide richness control in a closed loop, and on a continuous basis while the engine is operating, by using a so-called λ probe serving to detect the oxygen content in the exhaust gas from the engine, and connected to the engine control unit so as to guarantee delivery of an ideal air/fuel mixture, in particular when using three-function catalysts for which a stoichiometric mixture is required. Such closed loop richness control makes it possible to compensate in satisfactory manner for the dispersions in all of the components involved in measuring out the air-fuel mixture, and that might otherwise have an impact on performance in terms of controlling emission in the exhaust gas from the engine, if the above-mentioned dispersions were not compensated. The components concerned are those that serve to calculate the rate at which air is admitted to the engine and those that control the rate at which fuel is injected into the engine, so these components concerned include the injectors. However, unless using special strategies, closed loop richness control does not enable the characteristics of each of the components concerned to be identified whether individually or globally. In other words, the measuring out of the air-fuel mixture consists in controlling the rate at which air is admitted to the engine together with a corresponding fuel flow rate, with the closed loop richness control serving to compensate the ratio between the air flow rate and the fuel flow rate, but without identifying which part of the correction should be applied to the air flow rate or to the fuel flow rate, and in addition, richness control does not make it possible to calculate an individualized correction for each cylinder, and thus for each injector.
The problem on which the invention is based thus consists, starting from knowledge of a nominal or theoretical characteristic for the flow rate of an injector, in determining, in real time and as a function of injection control duration, the variation in said characteristic for at least one fuel injector of an engine, in order to determine the relationship that actually exists between the mass of fuel injected and the injection control duration for at least one injector under consideration, with this determination being done during training stages that are undertaken regularly while the engine is in operation on operating points that are not necessarily under steady conditions, and for training periods that are sufficiently short to avoid significantly degrading pollution emissions and to avoid perceptible discomfort for the occupants of the vehicle.
The training may relate not only to the flow rate characteristic of each of the injectors used in a given engine, but also to the average or global characteristic of all of the injectors of an engine under consideration, on the basis of a nominal or theoretical global characteristic defined by a nominal or theoretical global gain and a nominal or theoretical global offset, and also by a nominal or theoretical global non-linear zone.